As is known in the art, the deposition of a thin film or coating of, for example, carbon onto an optical waveguide fiber can reduce water corrosion of the fiber, as well as other types of chemically induced fatigue. In addition, such coatings are effective in reducing light attenuation resulting from absorption of hydrogen into the fiber from the environment.
Such thin films or coatings are generally referred to as hermetic coatings. As used herein, the term "hermetic coating" shall mean a thin film applied to a filament wherein the film is capable of absorbing light. Descriptions of these coatings and of apparatus and techniques for applying them to fibers as part of the fiber drawing process can be found in, for example, DiMarcello et al., U.S. Pat. No. 5,000,541, Ishiguro et al., U.S. Pat. No. 5,035,484, Schultz et al., U.S. Pat. No. 4,735,856, and Japanese Patent Publications 62-83,339 and 62-83,340. A discussion of a proposed technique for detecting defects in hermetic coatings appears in SBIR Phase I Final Report: Method for Detecting Pinholes in Hermetic Coatings of Optical Fibers, U.S. Army CECOM, Attn: AMSEL-RD-C3-LA-F, Fort Monmouth, N.J. 07703, Contract #DAAB07-91-C-B008, Topic #A90-219, TAI Inc., August 1991. The report also mentions measuring hermetic thicknesses.
In order to control the coating process, means must be provided for rapidly and reliably monitoring the thickness of the coating. A number of techniques have been disclosed in the art. Some have been based on the electrical properties of the hermetic coating. See Atkins et al., U.S. Pat. No. 5,013,130 and Kingsbury, U.S. Pat. No. 5,142,228. Others have employed an optical approach.
In particular, Frazee, Jr. et al., U.S. Pat. No. 4,952,226, discloses a system for monitoring the thickness of a carbon coating on a fiber with a polymer coating in which a laser beam is directed at the fiber and the intensity of the forward-scattered light is measured. As reported in this patent, the measured intensity is monotonically inversely proportional to the thickness of the carbon coating. As shown in Frazee's FIG. 5, the laser beam is split in two so that two intensity measurements can be made at right angles to one another.
Significantly, with regard to the present invention, the Frazee patent at column 4, lines 60-62, specifically teaches "eliminating the fine structure corresponding to interference of refracted and reflected rays." As discussed in detail below, the monitoring technique of the present invention is based on analyzing the interference pattern produced by the reflected and refracted rays, that is, the present invention relies on that which Frazee purposely eliminates.
A technique similar to the Frazee technique is disclosed in Inoue et al., "Development of Non-Contact Coating Thickness Monitor for Hermetically Carbon Coated Fiber," Conference Digest for the Proceedings of the Optical Fibre Measurement Conference, September 1991, York, England, pages 135-138. In this case, the forward scattered light is collected on a single photodiode (photodiode A in Inoue's FIG. 1), which as in Frazee, eliminates all of the fine structure from the scattering pattern.
Both the Frazee and Inoue techniques suffer from a number of disadvantages. One disadvantage involves the effects on the thickness measurement of fluctuations in the light source's power. In each case, a decrease in power will be interpreted as increase in coating thickness, and vice versa.
Inoue seeks to address this problem by including a second photodiode (photodiode B in his FIG. 1) to measure the power of what he refers to as the "reference light." The problem with this approach is that the optical fiber does not stay in one position as the drawing and hermetic coating process takes place, but rather moves around in the light beam. Since most laser source light beams have a Gaussian power distribution, this movement means that the fiber will be seeing different power intensities as a function of time. Inoue's photodiode B is fixed in space and thus only provides information about the average power of the beam, rather than the power of the light which has interacted with the fiber.
The Frazee apparatus includes rotatable cubes 45 and 46, each mounted on the shaft of a servo motor, for aligning Frazee's orthogonal laser beams with the fiber. See Frazee at column 5, lines 22-27. Frazee, however, does not disclose that the cubes are used to follow the changing position of the fiber as the drawing and coating process takes place. Also, Frazee does not provide a system for tracking overall power drift of his laser.
Another problem with the Inoue approach is illustrated in his FIG. 3. As shown therein, Inoue's attenuation factor increases in magnitude up to a coating thickness of around 800 angstroms and thereafter decreases in magnitude. That is, the function is double valued, i.e., the same attenuation value corresponds to two coating thicknesses. An ambiguity can thus arise in applying the technique to process control such that the controller may erroneously call for an increase in coating thickness where a decrease is actually needed.
A fundamental disadvantage of both the Frazee and Inoue approaches is that in each case the measured variable, total forward scattered light intensity, is a combination of reflected and refracted light whereas it is only the refracted light which contains information about the thickness of the coating since it is that light which has passed through the coating. That is, Frazee and Inoue look at an additive signal wherein the desired information in the refracted light is of low intensity relative to the undesired reflected light and is desensitized (swamped out). This is increasingly a problem as the coating gets thicker. As discussed below, the measured variable in the present invention, variation of fringe contrast, is substantially a direct function of the refracted light and thus is not susceptible to being swamped out by the reflected light.
The use of spatial frequency spectra to measure fiber diameters is discussed in an article by Mustafa A. G. Absuhagur and Nicholas George entitled "Measurement of optical fiber diameter using the fast Fourier transform," Applied Optics, Vol. 19, pages 2031-2033 (1980). This reference, however, contains no disclosure or suggestion that such spectra can be used to monitor the thickness of a hermetic coating on a fiber.